Embodiments of the invention relate generally to magnetic resonance (MR) imaging, and more particularly, to tissue-specific MR imaging of metabolites using spectral-spatially formed stimulated echoes.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which has a frequency near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The set of received nuclear magnetic resonance (NMR) signals resulting from a scan sequence are digitized and sent to a data processing unit for image reconstruction using one of many well known reconstruction techniques. It is desirable that the imaging process, from data acquisition to reconstruction, be performed as quickly as possible for improved patient comfort and throughput.
For some procedures and investigations, it is also desirable for MR images to display spectral information in addition to spatial information. The traditional method for creating such images is known as “chemical shift imaging” (CSI). CSI has been employed to monitor metabolic and other internal processes of patients, including imaging hyperpolarized substances such as 13C labeled contrast agents and metabolites thereof. In such 13C imaging, spectral composition is dependent not only on tissue type and health of that tissue, but also on the time the image is acquired relative to the injection of the hyperpolarized 13C agent. The hyperpolarization of contrast agents tends to have a very limited lifetime; typical T1 lifetimes are on the order of a few minutes in vivo.
While CSI, as a sequence for imaging hyperpolarized substances, provides valuable information on tissue type and health of that tissue, prior art CSI does not distinguish where or when metabolic products are formed in a 13C-hyperpolarized metabolic imaging acquisition. That is, while hyperpolarized 13C imaging of 13C-1-pyruvate and its' metabolic products lactate, alanine and bicarbonate can provide tissue-specific metabolic fingerprints (spectra), these fingerprints can be confounded by the local uptake of metabolic products not formed in the tissue of interest. For example, lactate in the blood stream formed by the heart or by red blood cells may be taken up by the tissue of interest, confounding the measurement of locally formed lactate.
It would therefore be desirable to have a system and method of MR imaging with spectral information and hyperpolarization that is able to separate systemic from local metabolic activity. Specifically, it would be desirable to excite and image hyperpolarized agents and metabolites thereof for a volume of interest, while effectively excluding metabolic products formed outside the volume of interest.